1. Field of the Invention
This invention relates to a low expansion transparent glass-ceramic suitable for use as various materials for which low thermal expansion characteristics and transparency are strictly required such, for example, as materials for optical parts, materials for large type telescope mirror blanks, materials for a ring laser gyroscope, materials for precision parts such as parts of a reference gauge for precision measurement and various electronic materials.
2. Description of the Related Art
Known in the art are low expansion transparent glass-ceramics formed by melting, heat-treating, and crystallizing a base glass of a SiO.sub.2 -Al.sub.2 O.sub.3 -Li.sub.2 O system containing a nucleating agent. These glass-ceramics generally contain .beta.-quartz solid solution as a main crystalline phase and have a coefficient of thermal expansion of (.alpha..times.10.sup.-7 /.degree.C.) within a range of from -10 to +10.
Recent tendencies in the industry are that higher and more strict characteristics as described below are required for the materials used for the above described purposes:
(1) The base glass can be molten and refined easily so that the glass-ceramic substantially has little or no cord, foam or inclusion and has an optical homogeneity of a high degree. PA1 (2) A coefficient of thermal expansion is 0.+-.5.times.10.sup.-7 /.degree.C. within the temperature range of from -60.degree. C. to +160.degree. C., and the maximal variation in a .DELTA.1/1 curve (variation in relative length) which is represented by expansion and contraction of a material with respect to temperature change is 10 ppm or below. PA1 (3) The crystal produced is fine and the glass-ceramic has an excellent transparency, particularly an excellent light transmissivity in the visible ray region. PA1 (4) If the glass-ceramic contains Na.sub.2 P and K.sub.2 O ingredients, ions of these ingredients will diffuse to adversely affect properties of the glass-ceramic in the course of various processing such as film formation, washing and the like. Besides, these ingredients will increase the value of the coefficient of thermal expansion and variation in .DELTA.1/1 of the glass-ceramic so that the glass-ceramic contains substantially little or no Na.sub.2 O and K.sub.2 O ingredients.
As one of the above described prior art low expansion transparent glass-ceramic, Japanese Patent Laid-open 77137/1991 discloses a low expansion transparent glass-ceramic product exhibiting the inherent thermal expansion behavior of a base glass of SiO.sub.2 -Al.sub.2 O.sub.3 -Li.sub.2 O-ZnO system containing TiO.sub.2 and ZrO.sub.2 ingredients as nucleating agents, and optionally containing other ingredients such as P.sub.2 O.sub.5, MgO, CaO, Na.sub.2 O, and K.sub.2 O. However, the MgO ingredient in these prior art glass-ceramics is lower than the minimal value specified by the present invention, besides there is no argument upon the particular properties within the extent restricted strictly by the present invention. Furthermore, all the glass compositions described in the examples of the specification contain Na.sub.2 O ingredient, and the crystallization temperature of which is also the higher temperature of 850.degree. C., so that this prior art glass-ceramic product does not meet the above described requirements. Moreover, although there is an argument as to the variation of a 1/1, the temperature range is narrower, i.e. from -60.degree. C. to +100.degree. C., so that the advantage to be attained by the conventional glass-ceramic product is insufficient, and hence the product does not meet also the above described requirement.
Furthermore, Japanese Patent Laid-open No. 92681/1994 discloses a transparent glass-ceramic of SiO.sub.2 -Al.sub.2 O.sub.3 -Li.sub.2 O-MgO-ZnO-BaO system containing TiO.sub.2 and ZrO.sub.2 ingredients as nucleating agents. There is, however, no argument upon particular properties including CaO ingredient in the extent restricted strictly by the present invention.
Moreover, the U.S. Pat. No. 5,064,460 discloses a blue transparent glass-ceramic of SiO.sub.2 -Al.sub.2 O.sub.3 -Li.sub.2 O-MgO system containing TiO.sub.2 and ZrO.sub.2 ingredients as nucleating agents. There is, however, no description of CaO and BaO ingredients.
Furthermore, the U.S. Pat. No. 3,928,229 discloses a transparent glass-ceramic for laser use of a SiO.sub.2 -Al.sub.2 O.sub.3 -Li.sub.2 O-ZnO-Na.sub.2 O system containing TiO.sub.2 and ZrO.sub.2 ingredients as nucleating agents and further containing Nd. However, there is no description as to a BaO ingredient. With respect to alkaline earth oxides (MgO, ZnO, CaO, BaO), no argument upon the particular properties within the extent restricted strictly by the present invention. Besides the disclosed glass-ceramic contains Na.sub.2 O ingredients, so that this prior art glass-ceramic does not meet the above described requirements of the present invention.
It is, therefore, an object of the present invention to provide a low expansion transparent glass-ceramic with improved properties, namely, having a reduced amount of 10 ppm or below in the maximal variation of .DELTA.1/1 curve (variation in relative length) within a wide temperature range, and having a mere excellent optical homogeneity owing to improved melting and refining properties of the base glass without containing Na.sub.2 O and K.sub.2 O ingredients.